docs: remove INTEGRATION_PLAN.md (merged into ps2_opengl_integration/PLAN.md)

The workspace-level ps2_opengl_integration/PLAN.md was previously a
symlink into this repo. It is now a real file in the workspace folder
holding the unified plan (also subsuming the former
openvcl/GENERIC_COMPILER_ROADMAP.md, PENDING.MD, and VSM_COST_REPORT.MD).
This per-repo copy is therefore redundant.

Author: fjtrujy